Other Aromatic Compounds

Cydopentadienyl Anion Here we use a linear anion for our hypothetical transfonnalion  

Both calculations and experiments confirm that the cyclic anion has a lower 7r-electron energy than its open-chain counterpart. Therefore the cydopentadienyl anion is classified as aromatic. 

Cyclooctatetraene For cyclooctatetraene we consider the following hypothetical transformation  

Here calculations and experiments indicate that a planar cyclooctatetraene would have higher 7r-electron energy than the open-chain octatetraene. Therefore, a planar form of cyclooctatetraene would, if it existed, be antiaromatic. As we saw earlier, cyclooctatetraene is not planar and behaves like a simple cyclic polyene. 

Calculations indicate that the 7r-electron energy decreases for the hypothetical transformation from the allyl cation to the cyclopropenyl cation below. What does this indicate about the possible aromalidty of the cyclopropenyl cation 

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HMO theory is named after its developer, Erich Huckel (1896-1980), who published his theory in 1930 [9] partly in order to explain the unusual stability of benzene and other aromatic compounds. Given that digital computers had not yet been invented and that all Hiickel s calculations had to be done by hand, HMO theory necessarily includes many approximations. The first is that only the jr-molecular orbitals of the molecule are considered. This implies that the entire molecular structure is planar (because then a plane of symmetry separates the r-orbitals, which are antisymmetric with respect to this plane, from all others). It also means that only one atomic orbital must be considered for each atom in the r-system (the p-orbital that is antisymmetric with respect to the plane of the molecule) and none at all for atoms (such as hydrogen) that are not involved in the r-system. Huckel then used the technique known as linear combination of atomic orbitals (LCAO) to build these atomic orbitals up into molecular orbitals. This is illustrated in Figure 7-18 for ethylene. 

The binding behaviour of benzene can be extrapolated to many other aromatic compounds such as naphthalene and benzene derivativesInterestingly, a large number of probe molecules contain aromatic rings and many of them will prefer the outer regions of micelles, whereas in bilayer systems, the same molecules prefer the interior of the aggregate ". Qearly these probes cannot be used to determine polarity of the micellar interior or the extent of water penetration therein . 

It has already been noted that, as well as alkylbenzenes, a wide range of other aromatic compounds has been nitrated with nitronium salts. In particular the case of nitrobenzene has been examined kinetically. Results are collected in table 4.4. The reaction was kinetically of the first order in the concentration of the aromatic and of the nitronium salt. There is agreement between the results for those cases in which the solvent induces the ionization of nitric acid to nitronium ion, and the corresponding results for solutions of preformed nitronium salts in the same solvent. 

The chemical oil contains ca 50 wt % naphthalene, 6 wt % tar acids, 3 wt % tar bases, and numerous other aromatic compounds. The chemical oil is processed to remove the tar acids by contacting with dilute sodium hydroxide and, in a few cases, is next treated to remove tar bases by washing with sulfuric acid. 

The addition product, C QHgNa, called naphthalenesodium or sodium naphthalene complex, may be regarded as a resonance hybrid. The ether is more than just a solvent that promotes the reaction. StabiUty of the complex depends on the presence of the ether, and sodium can be Hberated by evaporating the ether or by dilution using an indifferent solvent, such as ethyl ether. A number of ether-type solvents are effective in complex preparation, such as methyl ethyl ether, ethylene glycol dimethyl ether, dioxane, and THF. Trimethyl amine also promotes complex formation. This reaction proceeds with all alkah metals. Other aromatic compounds, eg, diphenyl, anthracene, and phenanthrene, also form sodium complexes (16,20). 

Among other aromatic compounds that have been tricyanovinylated are phenanthrene (23), o-alkylphenols (24), pyrrole (23), indoles (23,25), 2-meth5lfuran (26), azulenes (26,27), diazocyclopentadiene (28), and a variety of phenyUiydrazones (26). 

Miscellaneous Dyes. Other classes of dyes that stiU have some importance are the stilbene dyes and the forma2an dyes. Stilbene ( es are in most cases mixtures of dyes of indeterminate constitution that are formed from the condensation of sulfonated nitroaromatic compounds ia aqueous caustic alkah either alone or with other aromatic compounds, typically arylarnines (5). The sulfonated nitrostilbene [128-42-7] (79) is the most important nitroaromatic, and the aminoa2oben2enes are the most important arylarnines. Cl Direct Orange 34 [2222-37-6] (Cl 40215-40220), the condensation product(s) of (79) and the aminoa2oben2ene [104-23-4] (80), is a typical stilbene dye. 

Like other aromatic compounds, aromatic ethers can undergo substitution in the aromatic ring with electrophilic reagents, eg, nitration, halogenation, and sulfonation. They also undergo Eriedel-Crafts (qv) alkylation and acylation. 

Through a study of the influence of thiophene and other aromatic compounds on the retardation and chain transfer on the polymerization of styrene by stannic chloride, the relative rates of attack of a carbonium-ion pair could be obtained. It was found that thiophene in this reaction was about 100 times more reactive than p-xylene and somewhat less reactive than anisole. ... 

Although hydrotreating increases the percentage of sulfur in coke and slurry, the actual amount of sulfur is substantially less than in the nontreated feeds. Sulfur still plays a minor role in unit conversion and yields. Its affect on processing is minimal. Some aromatic sulfur compounds do not convert, but this is no different from other aromatic compounds. They become predominately cycle oil and slurry. This tends to lower conversion and reduce maximum yields. 

Simple aromatic hydrocarbons come from two main sources coal and petroleum. Coal is an enormously complex mixture made up primarily of large arrays of benzene-like rings joined together. Thermal breakdown of coal occurs when it is heated to 1000 °C in the absence of air, and a mixture of volatile products called coal for boils off. Fractional distillation of coal tar yields benzene, toluene, xylene (dimethylbenzene), naphthalene, and a host of other aromatic compounds (Figure 15.1). 

Chemists sometimes represent the two benzene resonance forms by using a circle to indicate the equivalence of the carbon-carbon bonds. This hind of representation has to be used carefully, however, because it doesn t indicate the number of tt electrons in the ring. (How many electrons does a circle represent ) In this book, benzene and other aromatic compounds will be represented by a single line-bond structure. We ll be able to keep count of tt electrons this way but must be aware of the limitations of the drawings. 

On page 132, atropisomerism was possible when ortho substituents on biphenyl derivatives and certain other aromatic compounds prevented rotation about the bond. The presence of ortho-substituents can also influence the conformation of certain groups. In 88, R= alkyl, the carbonyl unit is planar with the trans C=0 - F conformer more stable when X=F. When X=CF3, the cis and trans are planar and the trans predominates. When R = alkyl there is one orthogonal conformation but there are two interconverting nonplanar conformations when R=0-alkyl. In 1,2-diacylbenzenes, the carbonyl units tend to adopt a twisted conformation to minimize steric interactions. " ... 

Naphtha is a mixture of aliphatic hydrocarbons isolated from petroleum by distillation. When it is passed over a catalyst under the right conditions, carbon rings are formed, followed by the sphtting of hydrogen from the carbon rings to produce benzene, toluene, and other aromatic compounds. 

Analogous ring fission reactions have also been found in stndies on the metabolism of other aromatic compounds by the yeast Trichosporon cutaneum whose metabolic versatility is indeed comparable with that of bacteria. Examples inclnde the degradation of... 

Nozawa T, Y Maruyama (1988) Anaerobic metabolism of phthalate and other aromatic compounds by a denitrifying bacterium. J Bacteriol 170 5778-5784. 

Fredrickson JK, FJ Brockman, DJ Workmnan, SW Li, TO Stevens (1991) Isolation and characterization of a subsurface bacterium capable of growth on toluene, naphthalene, and other aromatic compounds. Appl Environ Microbiol 57 796-803. 

Gorny N, G Wahl, A Brune, B Schink (1992) A strictly anaerobic nitrate-reducing bacterium growing with resorcinol and other aromatic compounds. Arch Microbiol 158 48-53. 

The Shikimate pathway is responsible for biosynthesis of aromatic amino acids in bacteria, fungi and plants [28], and the absence of this pathway in mammals makes it an interesting target for designing novel antibiotics, fungicides and herbicides. After the production of chorismate the pathway branches and, via specific internal pathways, the chorismate intermediate is converted to the three aromatic amino acids, in addition to a number of other aromatic compounds [29], The enzyme chorismate mutase (CM) is a key enzyme responsible for the Claisen rearrangement of chorismate to prephenate (Scheme 1-1), the first step in the branch that ultimately leads to production of tyrosine and phenylalanine. 

A number of other aromatic compounds have been observed to undergo photoaddition to anthracene to yield products similar in structure to the anthracene dimer ... 

Benzene, aniline and other aromatic compounds give explosive gelatinous ozonides, among other products, on contact with ozonised oxygen. 

Compounds of the porphyrin series, like other aromatic compounds, have a strong tendency to aggregate in solution. In general, aggregation is favoured by increased concentration and by an increased proportion of the poor solvent in a mixed solvent system.67-70 The aggregate shows reduced fluorescence and reduced PDT activity with respect to the monomer.71... 

The method was also applied to the benzoylation of other aromatic compounds (Tab. 7.7). The benzoylation of benzene itself, volatile and less reactive, seemed more difficult to perform (Tab. 7.7, entry 4). Silyl-substituted aromatics reacted by ipso Si-substitution [77], and were less volatile. With trimethylsilylbenzene, benzoylation occurred with an overall yield higher than for benzene, but the competitive H-substitu-tion was also observed (entry 5). 

The electrogenerated radical anions of aromatic hydrocarbons, e.g. DPA, rubrene, fluorene, can also act as reductants towards electro-chemically obtained radical cations which are derivatives of other aromatic compounds such as N,N-dimethyl-/>-phenylenediamine (Wurster s red) 150> (see Section VIII. B.). When a mixture of DPA and a halide such as 99 (DPACI2) or 100 is electrolysed, a bright chemiluminescence is observed the quantum yields are about two orders of magnitude higher than that of the DPA radical anion-radical cation reaction 153>. 

Hz for the ortho, meta and/or para coupling constants of p-nitroanisole, two dinitrotoluenes and 2,4-dinitrochlorobenzene. Both reports indicate that other aromatic compounds investigated showed even less solvent dependence. 

Unless the immunoassay kit is benzene sensitive, the kit may display strong biases, such as the low affinity for benzene relative to toluene, ethylbenzene, xylenes, and other aromatic compounds. This will cause an underestimation of the actual benzene levels in a sample, and since benzene is often the dominant compound in leachates due to its high solubility, a low sensitivity for benzene is undesirable. 

To test the first hypothesis, solutions of 3,5-dinitroanisole and hydroxide ions were flashed and the absorption spectra at different time intervals after excitation were compared. The absorption ( max 400-410 nm) that remains after all time-dependent absorptions have decayed can be shown to be due to 3,5-dinitrophenolate anion, the photosubstitution product of 3,5-dinitroanisole with hydroxide ion. When the absorption band of the 550-570 nm species is subtracted from the spectrum of the solution immediately after the flash, there remains an absorption at 400-410 nm, which can also be ascribed to 3,5-dinitrophenolate anion. The quantity of this photoproduct does not increase during the decay of the 550-570 nm species. Therefore the 550-570 nm species cannot be intermediate in the aromatic photosubstitution reaction of 3,5-dinitroanisole with hydroxide ion to yield 3,5-dinitrophenolate. Repetition of the experiment with a variety of nucleophiles on this and other aromatic compounds yielded invariably the same result nucleophilic aromatic photosubstitution is, in all cases studied, completed within the flash duration (about 20jLts) of our classical flash apparatus. 

The extracts of three species of male North American decorator wasps, Eucerceris rubripes, E. conata and E. tricolor, were analyzed to reveal the presence of one major component in large quantities. This component detected in the head extract of males was identified as (Z )-3-hexenyl-3-hydroxybutanoate. The structure was confirmed by synthesis and the absolute configuration of the chiral center was determined to be R for the three species. In addition, 2- and 3-hexenoic acid and a few other aromatic compounds were also detected in varying quantities in males and females, along with hydrocarbons and fatty acids. 

Benzene is the fundamental aromatic compound. An understanding of the behavior of many other aromatic compounds is much easier if you first gain an understanding of benzene. For this reason, you may find it useful to examine a few key characteristics of benzene, which we discuss in the following sections. 

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